The advantageous mechanical properties, dimensional stability and optical properties of polyester films are well-known and are exploited in many areas of technology. Many adaptations of polyester films have been proposed in order to tailor the properties of a polyester film to a particular application. However, an adaptation which improves the performance of a polyester film in one respect may be detrimental to the performance of the film in another respect. For instance, a modification to improve the optical properties of a film may have a detrimental effect on the mechanical properties of the film. Accordingly, it is often difficult to obtain polyester films having a suitable combination of desirable properties.
One area in which polyester film has proved particularly useful is as a substrate in electronic display applications. In such applications, the substrates can be transparent, translucent or opaque, but are typically transparent. The substrates are usually required to meet stringent specifications for optical clarity, flatness and minimal birefringence. Typically, a total light transmission (TLT) of 85% over 400-800 nm coupled with a haze of less than 0.7% is desirable for displays applications. Surface smoothness and flatness are necessary to ensure the integrity of subsequently applied coatings. The substrates should also have good barrier properties, i.e. high resistance to gas and solvent permeation. The substrates should also exhibit chemical resistance. Mechanical properties such as flexibility, impact resistance, hardness and scratch resistance are also important considerations. A polymeric substrate should also exhibit good dimensional stability (for instance it should exhibit low shrinkage and low “curl”) when subjected to the processing conditions, particularly elevated temperature, used in the manufacture of electronic display devices. Plastic substrates are of lighter weight than glass or quartz sheets of equal thickness, and flexible plastic substrates have the further advantage that they allow the printing of electronic components onto the substrate in a reel-to-reel process, which reduces cost and allows the manufacture of curved-surface devices.
A variety of techniques have been developed to manufacture electronic displays (or screens). Of particular interest are flexible rollable displays (or screens), which are finding applications in cell-phones and handheld or palm-top computers (also known as PDAs). The rollable display may be stored in rolled form and unrolled to full size when desired, and then connected to a cell-phone or PDA. Manufacture of such displays begins with a polymeric laminate, which is processed by the application of a succession of layers or coatings to produce the final display. In one such process, the polymeric laminate is adhered, by means of an adhesive composition, to a rigid base (typically a glass or silicon wafer) in order to hold the laminate secure and flat while some or all of the subsequent layers e.g. the electronics are applied thereto. Desirably, the adhesive composition should (i) be capable of adhering the polymeric laminate to the rigid base and retaining that adhesive bond throughout the subsequent processing; (ii) be chemically, mechanically and thermally resistant; and (iii) allow peeling of the processed laminate away from the rigid base with a clean peel, leaving the adhesive coating on the rigid base. Thus, after processing, the plane of failure of the adhesive bond should be at the polymeric laminate/adhesive interface, rather than within the adhesive layer itself or at the adhesive/rigid base interface. Such adhesive compositions are referred to herein as “switchable adhesives”, which compositions may therefore be defined as exhibiting the essential feature of transferability between substrates which occurs as a consequence of a change in the plane of failure of the adhesive composition as a consequence of the processing to which the ensemble is subjected.
Thus, a switchable adhesive layer is one which is applied to a polymeric film substrate to form a laminate, which laminate is subsequently applied to a rigid substrate base such that the switchable adhesive layer is disposed adjacent the rigid base, wherein the switchable adhesive layer remains attached to the rigid base upon separation of the polymeric substrate from the rigid base, particularly wherein the rigid base is a glass or silicon wafer substrate and the polymeric film substrate comprises a polyester film.
To date, the use of such polyester materials in such a manufacturing process has been limited by the thermal stability of the adhesive composition. The currently used adhesive-coated films can be heated to temperatures of approximately 150° C. without any issues. While this was previously satisfactory, there is an increasing need to take processing temperatures higher than 150° C., which is a problem for conventionally used materials because they start to degrade and bubble at these higher temperatures. Bubbling is a problem in the manufacture of a smooth and even display.
Hence, there is a need for an adhesive-coated film which has the optical and mechanical properties to make it useful in the manufacture of electronic displays and which is also switchable but which can also withstand processing temperatures of greater than 150° C.